Dear Sujatha, when did you come to know about the approval for the extension of your tenure? You could have applied immediately for the extension of maternity leave. Anyway, even now you may apply for the extension of maternity leave. Since the delivery is already over, you may submit the certificate from the hospital authorities regarding your delivery. If you have obtained the birth certificate for the newborn child from the local municipal office, then you may attach that as well.
Please address your application to the head of the institute; however, route your application through the proper channel.
By the way, the heading of your post is misleading. Your maternity leave was sanctioned until the end of your tenure. Therefore, the authorities cannot be blamed per se for not adhering to the provisions of the Maternity Benefits Act, 1961. If you were not eligible to get maternity leave after 30th May 2019, then how could the approving authorities have sanctioned it? The institute authorities have done the best they could.
Thanks,
Dinesh Divekar